Surface mount technology has made it possible to densely populate both sides of a circuit board with semiconductor devices. Because leads of a surface mount integrated circuit (IC) package may be placed closer together than through-hole pins of a dual-in-line package (DIP), the component size of surface mount ICs has also shrunken down. These small packages have been termed small-outline IC packages and chip carriers. Chip carriers are typically used in applications that require large lead counts and employ a variety of mounting techniques, such as flatpacks, quad flatpacks (QFPs), J-leads, Gull-wing leads, leadless, and the more recently developed ball grid arrays (BGAs).
The ball grid array mounting technique uses rows and columns of closely positioned solder balls positioned on one side of the package as the outer leads of the integrated circuit. Ball grid array packages offer many advantages, including lower yield loss from bent leads and misregistration, higher throughput from greater placement tolerances and a more repeatable assembly process. However, a major concern with the ball grid array package is solder joint reliability between the ball grid array and the printed circuit board.
The solder joints formed between the ball grid array and the printed circuit board experience high thermal cyclic stress every time the device is powered on or off. As the device heats up, the different temperatures and coefficients of thermal expansion causes the device to expand at a different rate than the printed circuit board. Solder joint stress becomes even more critical with large ball grid arrays and/or where power dissipation is high.